The present invention relates to an ultra high molecular weight polyethylene composition particularly suited for making molded plastic articles of high strength and high porosity, to articles produced therefrom, and to a process for their production.
Ultra high molecular weight polyethylene (UHMW-PE) is known in the art to possess properties markedly superior to similar polyethylenes of lower molecular weight. Such properties include toughness, impact strength, abrasion resistance, anti-friction properties and resistance to corrosion and chemical attack. But because of its extremely high molecular weight (at least 10.sup.6) and the high viscosity of UHMW-PE melts, it is extremely difficult to process the material by conventional techniques such as injection molding, blow molding or screw extrusion. Such processing also may give rise to a degradation of the polymer chains with a consequential diminution of physical properties.
Porous sheets and articles made from polyethylene are known in the art. For example, U.S. Pat. No. 3,024,208 discloses a process for the production of porous polyethylene articles made by sintering (heating) of particles of polyethylene having a molecular weight of about 10,000 to 1,000,000 under heat and pressure without melting of the particles. U.S. Pat. No. 3,954,927 discloses a method for preparing porous articles comprising first forming a heterogeneous mixture of UHMW-PE particles with 50 to 95% by weight of a hydrocarbon such as mineral oil or a paraffin wax, heating the mixture to a temperature above the melting point of the hydrocarbon, forming the mixture into a shape, heating the shape to a second temperature above the melting point of the polyethylene until the polyethylene particles are completely fused, cooling the shape and then extracting the hydrocarbon to form the porous article. This extraction process leads to the formation of voids in the fused mass which creates porosity.
While these and similar processes offer the opportunity to minimize degradation of the polyethylene caused by conventional processing techniques as referred to above, it has been found that such porous articles are often brittle and easily fracturable, particularly when manufactured to give articles of higher porosity. Although low porosity articles may exhibit reasonably good stiffness, such low porosity articles are not suitable for many applications. Thus, there is a compromise of one property vs. the other. Accordingly, there is a continuing need to develop a process for producing porous articles made from UHMW-PE which offers better control of porosity while at the same time maintaining or improving the strength and flexibility of the article.